1. Field of the Invention
The present invention relates to a wave front aberration measuring apparatus for obtaining a spot image by focusing a laser beam from a laser source through an imaging lens, for use in an optical disk apparatus or a laser beam printer, for example, and more particularly to an apparatus for measuring astigmatism and coma aberration.
2. Description of the Prior Art
An optical system of a magneto-optical disk apparatus, for example, is one type of optical system as mentioned above. This optical system focuses a laser beam from a laser source upon the disk by an objective optical system, records information on the disk and reproduces information from the disk. A laser flux must be focused on the disk in a spot as small as the diffraction limit or so, in order to record and reproduce the information accurately. Additionally, the astigmatism or the coma aberration of the laser beam focused upon the disk must be reduced to be as small as possible.
When the wave front of the laser beam made incident into the disk has astigmatism, a focusing position in the vertical plane and a focusing position in the horizontal plane result in separation and the laser beam can not be sufficiently focused. Also, when the central axis of the laser beam is inclined relative to the optical axis of the objective optical system, a coma aberration will occur and the spot upon the disk will enlarge.
Accordingly, this kind of optical system requires both the measurement of the astigmatism and coma aberration of the beam entered into an object and correction of each aberration based on such measurement.
Japanese Patent Laid-open Publication No. SHO 61-109015 discloses the conventional correction method of astigmatism. The method mentioned in the Publication is that the wave front of a light beam having an astigmatism emitted from a laser diode is corrected before being made incident on an objective optical system by an anamorphic optical system.
However, an astigmatism generated by optical elements placed between the anamorphic optical system and the disk can not be eliminated by only the correction of an astigmatism of the light beam through the anamorphic optical system. In order to completely eliminate the astigmatism of the beam focused on the disk, the aberration generated by the optical element must be reduced. Since very high accuracy is required to make these optical elements placed between the anamorphic optical system and the disk, the costs of making them are great. The flattening of a mirror especially requires a very high accuracy because a mirror of low flatness (i.e., not precisely flat) generates a large astigmatism.
The conventional method for measuring the inclination of a central axis of a light beam and an optical axis of an objective optical system to measure a coma aberration is that a beam of a He-Ne laser, for example, is first made incident into an objective optical system perpendicularly to an optical disk, and then the beams reflected by each surface of the lenses comprising an objective optical system are interfered, and next the interference fringes are projected onto the screen, and finally the inclination of the objective optical system is measured.
However, in the conventional inclination measuring method of an objective optical system, the inclination of the optical axis of an objective optical system can not be measured with high accuracy because interference fringes occur by superposing all beams reflected from each surface of the lenses comprising the objective optical system, and the inclination or the dislocation of the axes of the lenses affects the interference fringes. Additionally, since this method requires a He-Ne laser apparatus, etc., its cost is large, when used as a single-purpose method of measuring an inclination.
With regard to the measurement of a coma aberration, a highly accurate analysis can be gained by computerizing an interference fringe using an interferometer such as ZYGO 8100 (a trade name of ZYGO Co. Ltd.). This method not only costs much but also can not obtain a real time result of the measurement. Therefore, it has been difficult to adjust the inclination of an objective optical system and at the same time to confirm the result of the measurement.